As an alternative to the conventional mechanical groove recording, optical density recording utilizing a laser beam has been proposed to record video and/or audio information on a photosensitive disk. Since this optical recording permits the use of a non-contact type transducer during playback, the problem of impairing the quality of recorded information can be completely eliminated. To permit high quality recording of an analog signal, the signal should first be quantized and each quantized signal is encoded into a series of binary signals. One approach that has been proposed is to record such binary signals on a bit-by-bit basis. This requires that the signals be recorded on a track as wide as 1 micrometer, and consequently involves the use of a costly mechanism for playback to provide high precision tracking and focusing of beam onto the desired track. Because of the narrow width track, the recorded information is less immune to the effects of dust and scratches, a problem which somewhat offsets the advantages of the optical density recording.
Fourier transform holography has been considered as a solution to such problems. To provide recording of a Fourier transform hologram binary signals are converted into a two-dimensional pattern of binary optical densities and the interference fringe pattern of such optical information is recorded as a unit hologram on a photographic medium. It is however necessary that the recording medium be substantially motionless during exposure to the incident laser beam in order to obtain interference fringes of a tolerable degree of sharpness. It has been found that distance travelled by the recording medium during the exposure must be kept below one eighth of the spacing between successive fringes of the hologram being recorded. One approach that has been proposed to meet this requirement employed one-dimensional Fourier transform holography. The one-dimensional hologram is the record of a series of laterally spaced strip-like patterns successively arranged along the track and each strip-like pattern extending across the width of the track represents the Fourier spectrum while the optical density of each strip varies with the length of the track to represent the binary information. This method of recording can be regarded as a compromise between bit-by-bit direct recording and two-dimensional hologram recording because of its resemblance to the former in terms of the time-varying component of the recorded information and its likeness in someway to the latter in terms of the formation of interference fringes. However, it falls short of the latter in terms of the redundancy of information and the tolerance in precision required of the focusing and servo mechanisms.
The concept of recording a series of two-dimensional Fourier transform holograms has been precluded by the fact that the above-mentioned requirement can only be met if an extremely long period of time is allowed for recording, using methods conventionally available.